Conventionally, surface acoustic wave filters have been widely used as frequency selection filters (hereinafter, referred to also as “filter”) to be used for the RF (radio frequency) stage of mobile communication apparatuses such as mobile telephones and automobile telephones. In general, desired characteristics required for the frequency selection filter are a wider pass band, a lower loss and a higher attenuation. In recent years, there have been strong demands for a lower insertion loss in a surface acoustic wave filter so as to further improve the receiving sensitivity and achieve lower power consumption in a mobile communication apparatus. Moreover, recently, built-in antennas using dielectric ceramics or the like have come to be used in place of conventional whip antennas so as to achieve a smaller size of the mobile communication apparatus. For this reason, it becomes difficult to obtain a sufficient gain in the antenna, and from this point of view also, there have been stronger demands for further improving the insertion loss of a surface acoustic wave filter.
In order to achieve such a wider band pass and lower insertion loss, a double mode surface acoustic wave resonator filter has been proposed in which, for example, three IDTs (Inter Digital Transducer) are installed on a piezoelectric substrate so that longitudinal primary mode and longitudinal tertiary mode are utilized.
In particular, an arrangement has been proposed in which by placing narrow pitch portions of electrode fingers on end portions of adjacent IDTs, radiation loss of bulk wave among IDTs is reduced to control the state of the resonance mode so that a wider pass band and a lower insertion loss are achieved (for example, see JP-A No. 2002-9587).
Moreover, in recent years, the number of applied parts has been cut in order to achieve small size, light weight and low cost of a mobile communication apparatus and the like, and there have been demands for adding new functions to a surface acoustic wave filter. One of the demands is to achieve a construction of an unbalanced input/balanced output type or a balanced input/unbalanced output type. Here, the balanced input or the balanced output refers to a construction in which a signal is inputted or outputted as a potential difference between two signal line paths, and the signals of the respective signal line paths have the same amplitude and opposite phases. In contrast, the unbalanced input or the unbalanced output refers to a construction in which a signal is inputted or outputted as a potential of one line path relative to the ground potential.
In general, a conventional surface acoustic wave filter is an unbalanced input/unbalanced output type surface acoustic wave filter (hereinafter, referred to as an unbalanced type surface acoustic wave filter); therefore, when a circuit or an electronic part of a balanced input type is connected to the succeeding stage of the surface acoustic wave filter, a circuit construction in which an unbalance/balance converter (hereinafter, referred to as “balun”) is interpolated between the surface acoustic wave filter and the circuit or the like on the succeeding stage has been adopted. In the same manner, when a circuit or an electronic part of a balanced output type is placed on the preceding stage of the surface acoustic wave filter, a circuit construction in which a balun is interpolated between the circuit or the like on the preceding stage and the surface acoustic wave filter has been adopted.
At present, in order to eliminate the balun, an unbalanced input/balanced output type surface acoustic wave filter or a balanced input/unbalanced output type surface acoustic wave filter (hereinafter, referred to as a balanced type surface acoustic wave filter), which allows a surface acoustic wave filter to have an unbalance/balance conversion function or a balance/unbalance conversion function, has been developed for practical use. In order to satisfy the demands for the unbalance/balance conversion function, a longitudinal coupling type double mode filter has been widely used. Moreover, with respect to the RF filter, those filters having one of the connection terminals unbalance-connected with its input/output impedance matching 50Ω, while the other is balance-connected, with its input/output impedance matching 100 to 200Ω, are required in most cases.
FIG. 17 is a plan view that schematically shows an electrode structure of a surface acoustic wave device 1000 serving as a conventional surface acoustic wave filter having the balance/unbalance conversion function. In the surface acoustic wave device 1000, a surface acoustic wave element 1012 and a surface acoustic wave element 1013 are placed on a piezoelectric substrate 1001. The surface acoustic wave element 1012 is configured by three IDTs 1002, 1003 and 1004 and reflectors 1008 and 1009 that are placed on the two sides thereof. The surface acoustic wave element 1013 is configured by three IDTs 1005, 1006 and 1007 and reflectors 1010 and 1011 that are placed on the two sides thereof.
The surface acoustic wave element 1012 and the surface acoustic wave element 1013 are parallel-connected to an unbalanced signal terminal 1014. Upon application of an electric field to the mutually opposing comb-shaped electrodes, each of the IDTs 1002, 1004, 1005 and 1007 is allowed to excite a surface acoustic wave. The surface acoustic wave thus excited is propagated to the IDT 1003 in the center of the surface acoustic wave element 1012 and the IDT 1006 of the surface acoustic wave element 1013. Here, the phase of the IDT 1003 forms a opposite phase different from the phase of the IDT 1006 by 1800, and the signal is finally transmitted from one of the comb-shaped electrodes of the IDT 1003 and 1006 to the balanced output signal terminals 1015 and 1016, and balance-outputted. With this structure, the balance/unbalance conversion function can be achieved.
FIG. 18 is a plan view that schematically shows an electrode structure of another conventional surface acoustic wave device 2000. As shown in FIG. 18, in the surface acoustic wave device 2000, with respect to a longitudinal coupling double mode filter on the first stage (on the upper stage side in the Figure) having three IDTs 2002, 2003 and 2004 sandwiched by reflectors 2010 and 2011 on both of the sides, an unbalanced terminal 2021 is connected to the IDT 2003 in the center, and the IDTs 2002 and 2004 on the two sides are respectively longitudinally connected to IDT 2005 and IDT 2007 on the second stage. Moreover, an IDT 2006 in the center of the second stage (on the lower stage side in the Figure) is divided into two, and respectively connected to balanced signal terminals 2022 and 2023 with inverted phases. Thus, it is possible to achieve a balance/unbalance conversion function (for example, see JP-A No. 11-97966).
Moreover, with respect to a surface acoustic wave filter of a resonator type using a conventional longitudinal coupling double mode filter, a structure has been proposed in which among three IDTs placed side by side along a propagation direction of a surface acoustic wave, the IDT placed in the middle is allowed to have an even number of pairs of electrode fingers, with the polarities of adjacent electrode fingers being inverted to each other, so as to improve the degree of balance between the amplitude and the phase (for example, see JP-A No. 2002-84164). Here, with respect to the degree of balance of the amplitude and of the phase, in the case when a signal is inputted or outputted as a potential difference between two signal line paths, as the sizes of the amplitudes of signals between the respective signal line paths become closer to each other, it is said that the degree of balance of the amplitude becomes more superior, and as the difference in the phases of the respective signals becomes closer to 180°, it is said that the degree of balance of the phase becomes more superior.
By using a conventional surface acoustic wave filter as shown in FIG. 17, the unbalance/balance conversion function can be achieved. However, such a surface acoustic wave filter has a problem in that, depending on selection of polarities of electrode fingers adjacent to each other between adjoining IDTs (depending on the combination of polarities of the respective electrode fingers), fine ripples occur within the pass band in terms of filter characteristics (frequency characteristics) to cause degradation in the insertion loss. FIG. 19 is a drawing that exemplifies the frequency characteristic near the pass band of such a conventional surface acoustic wave filter. In FIG. 19, such a fine ripple is indicated by an arrow portion.
Moreover, conventionally, with respect to the means for realizing a surface acoustic wave filter with a higher attenuation outside the pass band, a method has been widely used in which a plurality of stages of longitudinal coupling surface acoustic wave elements, each having three IDTs placed closely with one another along the propagation direction of a surface acoustic wave with reflectors being placed on the two sides thereof are longitudinally connected to construct a surface acoustic wave filter. Although the use of this structure makes it possible to increase the attenuation outside the pass band, the insertion loss within the pass band tends to deteriorate. For this reason, in an attempt to obtain a surface acoustic wave filter with a wider pass band width by using this structure, the required insertion loss is not sufficiently achieved.
In the case when a narrow pitch portion is formed at an end portion of an IDT as shown in a surface acoustic wave device disclosed in JP-A No. 2002-9587, since a portion having different electrode finger pitches is present in a state with a coupled surface acoustic wave, the ripple in the filter characteristic of the pass band becomes bigger, resulting in degradation in the shoulder characteristic. For this reason, it is not possible to obtain the flatness in the filter characteristic in the pass band. Moreover, only forming the narrow pitch portion at the end portion of the IDT causes a limitation of the number of basic resonance modes to be utilized for exciting the surface acoustic wave to a longitudinal primary mode and a longitudinal tertiary mode, with the result that the degree of freedom in designing becomes smaller because no other resonance modes can be utilized. Consequently, this method is insufficient in improving the flatness in filter characteristics in the pass band as well as in improving the insertion loss, with a wider pass band being provided.
In contrast, in a surface acoustic wave filter having a balance/unbalance conversion function, there have been demands for improvements in the degree of balance of the amplitude and of the phase within a pass band. For example, in a resonator-type electrode pattern in which reflectors are placed on the two ends of a surface acoustic wave transmitting path of a plurality of IDTs that are aligned side by side, so as to effectively resonate the surface acoustic wave, there have been demands for improving the degree of balance of the amplitude and of phase within the pass band.
JP-A No. 2002-9587 has disclosed a surface acoustic wave device which has a two-stage structure in which balanced input (output) terminals are connected to an IDT in the center on the second stage; however, since a structure having a modified structure of pitches of the IDTs located on the two sides of the IDT in the center or the like and a structure having a modified distance between the IDT located in the center and IDTs located on the two sides thereof are adopted so as to reverse the phase, the resulting problem is degradation in the degree of balance.
Moreover, in the case of a conventional resonator-type surface acoustic wave device 2000 disclosed in JP-A No. 11-97966, shown in FIG. 18, the structures, such as the numbers of opposing comb-shaped electrodes, the layout positions thereof, the polarities of electrode fingers mutually adjacent to each other between adjoining IDTs and the peripheral electrode patterns that cause to generate a parasitic capacity, are different from each other between IDTs 2003 and 2006. Consequently, since the amplitudes of signals to be transmitted to balanced output signal terminals 2022 and 2023 are different from each other, and since the phase is offset from the opposite phase, the resonator-type surface acoustic wave device 2000 fails to provide a sufficient degree of balance.
Moreover, in the surface acoustic wave filter disclosed in JP-A No. 2002-84164, since the polarity of the outermost side electrode finger of the IDT in the center and the polarity of the outermost side electrode finger of an adjacent IDT are different from each other on the right and left sides, the parasitic capacities formed on the respective balanced signal terminals are different from each other, with the result that the degree of balance is not necessarily improved sufficiently.